Capturing and converting solar photons into electrical current is an inherently inefficient process in semiconductor-based photovoltaic (PV) materials with well-defined band gaps, which correspond to the energy separation between the conduction band and the valence band. To generate charge carriers, photons must have energies larger than the band gap, whereas photons with energies below a material's band gap will generally pass through and not be absorbed. Photon energy in excess of the band gap is lost, and is converted into heat by thermalization. The energy of the photons in the terrestrial solar spectrum spans from about 0.5 to about 4.0 eV with a maximum intensity centered at about 2.6 eV. Thus, for a single photovoltaic material only a small part of the solar spectrum is efficiently absorbed to create charge carriers, with a large fraction of the spectrum wasted by heat generation. Heating and charge transport losses (due to phonon-carrier scattering, annihilation, and scattering of charge carriers at defects) limit efficiencies of single p-n junction photovoltaic devices to about 24% (the maximum theoretical efficiency is about 31%). The average efficiency of currently marketed products is about 12-18%. For multi-junction PV devices based on materials having different band gaps, the highest demonstrated efficiency to date is just over 40%, far below the theoretical 73% efficiency limit.
For the true realization of efficient photovoltaics, the fundamental processes of light absorption over a broad spectral range, carrier generation, carrier separation, and carrier extraction and transport must be simultaneously optimized. The first two processes are heavily influenced by the band structure of the photovoltaic material. The photovoltaic material quality (crystal structure, grains, doping, etc.) determines the efficiency of the other processes.
Over the last decade, the group III-nitrides (GaN, AlN, InN and their alloys) have become one of the most important new classes of semiconductor materials since Si. Indium nitride (InN) was recently discovered to have a band gap of about 0.7 eV. When alloyed with gallium nitride (GaN, band gap 3.4 eV) to produce In1-xGaxN thin film alloys (where 0≦x≦1) the material would have direct band gaps tunable from about 0.7 to 3.4 eV, which roughly covers the entire terrestrial solar spectrum. However, despite spectacular advances in In1-xGaxN film growth for LED device fabrication over the last decade, growing In1-xGaxN heterojunctions with an indium or gallium content spanning the entire composition range of 0-100% (and thus suitable for capturing light from the entire solar spectrum) has heretofore not been demonstrated.
InN has a low decomposition temperature (around 550° C.), making it incompatible with high-temperature growth methods such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Previous attempts at making a wide range of compositions of In1-xGaxN films have employed MBE and have met with very limited success, especially for high In-content materials (In greater than ˜20%). Using MBE for making compositionally graded In1-xGaxN films spanning the entire composition range of 0% to 100% has, in fact, not been possible, nor been demonstrated prior to this work. In general, high-temperature growth conditions (e.g. >850° C. for GaN and >600° C. for InN) present enormous technological barriers for making In-rich In1-xGaxN-based materials, prohibit the use of inexpensive substrates, and introduce problems with materials stability, composition, phase segregation (e.g., In tends to form “clusters” at high temperatures), and p-type doping.
There exists a need, therefore, for photovoltaic devices made from materials with bandgaps that are tunable over a broad spectral range, that exhibit exceptional photostability, are chemically/thermally stable, are environmentally benign, are radiation tolerant, and are relatively inexpensive. To this end, an additional need exists to optimize the growth of high-quality group-HI-nitride semiconductor materials over a broad composition range for use in photovoltaic devices. These same thin film materials would also find general use as solid state light emitting diodes covering the full visible spectral range with high efficiencies, and in other applications.